1. Field of the Invention
The present invention relates to a motor for an electric power steering assembly for assisting the steering force of an automotive steering wheel.
2. Description of the Related Art
FIG. 14 is a cross-section of a conventional electric power steering assembly. The electric power steering assembly comprises: an electric power steering assembly motor 1 (hereinafter "electric motor") for generating a rotational torque; and an electromagnetic clutch 2 connected to the electric motor 1 for engaging and disengaging the rotational torque from the electric motor 1.
The electric motor 1 comprises: a cylindrical yoke 3; permanent field magnets 4 secured facing each other inside the yoke 3 to form a 4-pole magnetic field portion; a shaft 7 supported inside the yoke 3 by a first bearing 5 and a second bearing 6 so as to be able to rotate freely; an armature 8 secured to the shaft 7; a commutator 9 secured to one end of the shaft 7; brushes 11 placed in contact with the surface of the commutator 7 by the elastic force of springs 10; a brush holder 12 for holding the brushes 11; a non-magnetic aluminum housing 14 connected to the yoke 3 by a clamping bolt 19 for securing the brush holder 12 by means of a brush holder clamping bolt 13; and a grommet 16 through which a lead wire 15 passes.
The armature 8 comprises: a core 17 having twenty-two slots extending in the axial direction; and a winding 18 constructed by winding wiring into the slots by a lap winding method.
The electromagnetic clutch 2 comprises: a clutch stator 20 secured to the housing 14 by a clutch clamping bolt 21; a clutch coil 22 disposed within the clutch stator 20; a boss 23 disposed on the end of the shaft 7 by means of the second shaft 6 so as to be able to rotate freely; a drive rotor 24 secured to the shaft 7; and a doughnut-shaped disk 25 secured to the boss 23 by means of a spring member 26.
In an electric power steering assembly of the above construction, the armature 8 is rotated together with the shaft 7 by electromagnetic action due to an electric current supplied to the winding 18 by means of the brushes 11 contacting the commutator segments 9.
At the same time, an electromagnetic circuit A is formed among the clutch stator 20, the drive rotor 24, and the disk 25, each being composed of magnetic materials, by passing an electric current through the clutch coil 22. The spring member 26 is stretched towards the clutch stator 20 side by the action of this electromagnetic circuit, whereby the disk is attracted and fastened to the drive rotor 24 and the drive unit 24 becomes integrated with the boss 23.
Consequently, because the drive rotor 24 is fastened to the shaft 7, the torque from the shaft 7 is transmitted to the drive rotor 24, the boss 23, and a worm shaft (not shown) bound by splining to the boss 23, thereby assisting the steering force on the steering wheel.
Furthermore, if a shock from the wheels acts on the electric power steering assembly while the torque assisting the steering force is being transmitted, slippage occurs between the disk 25 and the drive rotor 24, and the boss 23 is rotated relative to the shaft 7, absorbing the shock.
One problem with the above electric motor is that differences may arise in the electromotive force induced within the circuits of the winding 18 of the armature 8 due to imbalances in the electromagnetic circuit of the yoke 3, eccentricities in the armature 8, non-uniform current flowing through the brushes 11, etc., giving rise to circulating currents within the winding 18 flowing through the brushes 11, and as a result the deterioration of the commutating action of the brushes 11 and the increase in commutation sparks generated by the brushes 11 cause an increase in temperature, lead to a shortened working life of the brushes 11 and the commutator 9, and increase torque ripples.
FIG. 15 is a winding diagram for an electric motor 42 wherein commutator segments 30 of a commutator 9 which should have the same electric potential are electrically connected to each other using equalizing connectors 31 to prevent the occurrence of the above circulating currents, and FIG. 16 is an electric circuit diagram for the winding diagram in FIG. 15.
Equalizing connectors 31 connect each of the commutator segments 30 such that:
a first commutator segment 30a is connected to a twelfth commutator segment 30l; PA1 a second commutator segment 30b is connected to a thirteenth commutator segment 30m; PA1 a third commutator segment 30c is connected to a fourteenth commutator segment 30n; PA1 a fourth commutator segment 30d is connected to a fifteenth commutator segment 30o; PA1 a fifth commutator segment 30e is connected to a sixteenth commutator segment 30p; PA1 a sixth commutator segment 30f is connected to a seventeenth commutator segment 30q; PA1 a seventh commutator segment 30g is connected to an eighteenth commutator segment 30r; PA1 an eighth commutator segment 30h is connected to a nineteenth commutator segment 30s; PA1 a ninth commutator segment 30i is connected to a twentieth commutator segment 30t; PA1 a tenth commutator segment 30j is connected to a twenty-first commutator segment 30u; and PA1 an eleventh commutator segment 30k is connected to a twenty-second commutator segment 30x.
Furthermore, these figures show a first brush 11a in contact with the first commutator segment 30a and the second commutator segment 30b, and a second brush 11b in contact with the sixth commutator segment 30f, the seventh commutator segment 30g, and the eighth commutator segment 30h. They also show a third brush 11c in contact with the twelfth commutator segment 30l and the thirteenth commutator segment 30m, and a fourth brush 11d in contact with the seventeenth commutator segment 30q, the eighteenth commutator segment 30r, and the nineteenth commutator segment 30s.
FIG. 17 is an electric circuit diagram for an electric motor 40 comprising a lap winding, six poles, twenty-four slots, and six brushes 35a to 35f. Commutator segments 33a to 33z which should have the same electric potential are electrically connected to each other using equalizing connectors 34. A winding diagram for the motor for an electric power steering assembly in FIG. 17.
FIG. 18 is a winding diagram for an electric motor 41 comprising a lap winding, four poles, twenty-two slots, and four brush main bodies 36a to 36d. Commutator segments 38a to 38x which should have the same electric potential are electrically connected to each other using equalizing connectors 37. Each of the brush main bodies 36a to 36d comprises three brush portions 39.
In an electric motor 42 of the above construction, there are four brushes 11, leading to problems of torque loss and loud brush noise due to friction and resistance to the sliding of the brushes 11.
Furthermore, another problem is that when the number of brushes 11 is large, the probability that the contact between the brushes 11 and the commutator segments 30 will become unstable is that much greater, thereby increasing torque ripples and making the feel of the steering wheel unpleasant to the driver.
In addition, in the electric motor 42 above, the first brush 11a and the third brush 11c on the positive brush side have normally have the same electric potential as each other, and an equivalent electric current should flow through the first brush 11a and the third brush 11c. However, because currents of different amperage may flow through the first brush 11a and the third brush 11c due to irregularities in the voltage drop and resistivity of the contacts at the first brush 11a and the third brush 11c, irregularities in the resistance in the wires leading to the first brush 11a and the third brush 11c, etc., the brush size of the first brush 11a and the third brush 11c on the positive brush side must be designed with the magnitude of their respective current share in mind, which means that the size of the motor 42 cannot be reduced.
The electric motor 40 comprising a lap winding, six poles, twenty-four slots, and six brushes 35a to 35f suffers from the same problems.
Furthermore, in the electric motor 41 shown in FIG. 18, although the influence of the above irregularities is reduced because each of the brush main bodies 36a to 36d comprises three brush portions 39, in practice, this construction is only adopted in large electric motors and cannot be adopted in a motor for an electric power steering assembly requiring reduced size.